The invention relates to an MR imaging method for forming an image of the nuclear magnetization distribution in an examination zone (FOV), which method includes the following steps:    a) forming a first MR imaging sequence with a selectable minimum number of phase encoding steps,    b) measuring at least two separate MR signal data sets by means of at least two MR receiving coils,    c) reconstructing a first MR image by Fourier transformation and combination of the separate MR signal data sets while taking into account the respective spatial sensitivity profiles of the MR receiving coils.
The invention also relates to an MR apparatus for carrying out the method as well as to a computer program for controlling an MR apparatus.
It is known that magnetic resonance tomography is a spectral imaging method in which the nuclear magnetization is localized on the basis of the respective associated resonant frequency of the spins by using a spatially inhomogeneous magnetic field (magnetic field gradients). For imaging it is common practice to acquire the magnetic resonance signal in the time domain as a voltage, induced in a coil surrounding the examination zone, under the influence of a suitable sequence of RF pulses and gradient pulses. The actual image reconstruction is then performed by Fourier transformation of the time signals. The number, the distance in time, the duration and the strength of the gradient pulses used govern the scanning of the reciprocal so-called k space which determines the examination zone to be imaged (FOV or field of view) as well as the image resolution. A customary pulse sequence as used for the sequential scanning of the k space is, for example, the EPI (echo planar imaging) sequence. Requirements imposed on the image format and the image resolution govern the number of phase encoding steps and hence the duration of the imaging sequence. This is a direct cause of one of the essential drawbacks of magnetic resonance tomography, because the acquisition of an image of the complete examination zone with a resolution which suffices for diagnostic purposes usually takes an undesirably long period of time.
A large number of technical developments in the field of magnetic resonance tomography aim to achieve a drastic reduction of the image acquisition times. Further hardware developments, enabling as fast as possible switching of the magnetic field gradients, have met the limits of technical feasibility and the limits of what can be physiologically tolerated for a patient. However, for a number of applications, notably also for interventional radiology, the acquisition times are still too long.
The recent advent of parallel MR imaging methods such as, for example, the SMASH technique (Simultaneous Acquisition of Spatial Harmonics, see Sodickson et al., Magn. Reson. Med. 38, 591, 1997) or the SENSE technique (Sensitivity Encoding, see Pruessmann et al., Magn. Reson. Med. 42, 952, 1999) seems to enable a shift of the existing technical and physiological speed limits of conventional Fourier imaging. Such techniques are based on the recognition of the fact that the spatial sensitivity profile of the receiving coils impresses on the magnetic resonance signal position information which can be used for the image reconstruction. The parallel use of a plurality of separate receiving coils, each having a different spatial sensitivity profile, enables, by combination of the respective magnetic resonance signals detected, the acquisition time for an image to be reduced, in comparison with the conventional Fourier image reconstruction, by a factor which in normal cases is equal to the number of receiving coils used.
An MR imaging method of the kind set forth is disclosed, for example, in WO 99/54746. According to this known method, first a plurality of MR signal data sets is acquired while utilizing a plurality of receiving coils, an as small as possible number of phase encoding steps is then chosen so as to achieve a high image acquisition rate by scanning the k space only partly. The sensitivity of each of the receiving coils is dependent essentially on the distance between the nuclear magnetization to be detected and the relevant coil as well as on the geometry of the coil and on other factors such as, for example, the condition of the body of the patient to be examined. The spatial sensitivity profiles are used upon combination of the individual image data sets so as to reconstruct an image of the nuclear magnetization distribution in the examination zone while compensating for the inadequate scanning of the k space.
The known parallel MR imaging methods, however, have a drawback in that the signal-to-noise ratio of the reconstructed images, being approximately proportional to the square root of the image acquisition time in conventional non-parallel MR imaging techniques, is comparatively poor because of the fast measurement carried out by means of only a small number of phase encoding steps. The signal-to-noise ratio, moreover, may also be degraded by the respective position and geometry of the MR receiving coils used. In parallel MR imaging methods the inherently low sensitivity of magnetic resonance methods, therefore, does not enable a realistic reduction of the image acquisition time by more than a given measure while still maintaining an image quality which is acceptable for diagnostic purposes.
Recently MR imaging methods have become known which enable images to be reconstructed from MR signal data sets acquired in parallel, the phase encoding steps being selectable practically at random (see Kyriakos et al., Magn. Reson. Med. 44, 301, 2000). These known methods, however, also involve the problem that the image quality obtained is dependent to a high degree on the scanning of the k space and hence on the choice and the number of phase encoding steps. It appears from the cited article by Kyriakos et al. that it is difficult and intricate to specify, already before the beginning of the actual image acquisition, a suitable set of phase encoding steps whereby an optimum compromise of measuring time and image quality is achieved for the relevant measuring situation. The ideal parameters of the MR imaging sequence become apparent only from a series of test measurements. In real medical measuring situations, however, for lack of time, it is usually not possible to spend much time on optimizing the parameters of the imaging sequence prior to the actual image acquisition. Therefore, for real medical applications the known parallel MR imaging methods are of limited practical use only since the advantage of a gain in as achieved by the parallel data acquisition is lost again due to the time-consuming determination of the ideal phase encoding steps.
Based on the described state of the art, it is an object of the present invention to provide an improved parallel MR imaging method which eliminates said drawbacks and can be readily carried out by customary MR apparatus while requiring few adaptations only.